Floating-zone melting apparatus

ABSTRACT

APPARATUS FOR REFINING A ROD ACCORDING TO THE FLOATINGZONE MELTING TECHNIQUE INCLUDES ROTARY SHAFT MEANS ADAPTED TO CARRY AND ROTATE A ROD DURING SUBJECTION THEREOF TO THE FLOATING-ZONE MELTING TECHNIQUE, PLAY-FREE WORMDRIVE MEANS OPERATIVELY CONNECTED TO THE ROTARY SHAFT MEANS FOR ROTATING THE LATTER, CARRIAGE MEANS CARRYING THE ROTARY SHAFT MEANS, BALL-BEARING GUIDE MEANS GUIDING THE CARRIAGE MEANS FOR MOVEMENT, AND ROTARY BALL-BEARING SPINDLE MEANS OPERATIVELY CONNECTED TO THE CARRIAGE MEANS FOR DISPLACING THE LATTER, SO THAT THE ROD IS MAINTAINED FREE OF MECHANICAL VIBRATIONS DURING TREATMENT OF THE ROD ACCORDING TO THE FLOATING-ZONE MELTING TECHNIQUE.   D R A W I N G

July 13,1971 G EDER ETAL FLOATING-ZONE MELI'ING APPARATUS Original Filed 001;. 22. 1965 3 Sheets-Sheet l FigI PLAY- FREE SLIDE BEARING VIBRATOR AUXILIARY MASS SYSTEM (DAMPER) DRIVE SHAFT FOR ROD.

CAR RAGE PLAY FREE WORM DRIVE July 13, 1971 E ETAL 3,592,611

FLOATING-ZONE MELTING APPARATUS Original Filed Oct. 22, 1965 3 Sheets-Sheet 5 Fug 2a SHOCK ABSORBER OR \l.lr.l-l I1. W l ax ww n a 1. 1. l.

4] fig 7C] SHOCK ABSORBER OR DASHPOT MOUNTED United States Patent int. Cl. B613 17/10 US. Cl. 23-273 8 Claims ABSTRACT OF THE DISCLOSURE Apparatus for refining a rod according to the floatingzone melting technique includes rotary shaft means adapted to carry and rotate a rod during subjection thereof to the floating-zone melting technique, play-free wormdrive means operatively connected to the rotary shaft means for rotating the latter, carriage means carrying the rotary shaft means, ball-bearing guide means guiding the carriage means for movement, and rotary ball-bearing spindle means operatively connected to the carriage means for displacing the latter, so that the rod is maintained free of mechanical vibrations during treatment of the rod according to the floating-zone melting technique.

This is a division of our copending application Ser. No. 501,690, filed Oct. 22, 1966, and abandoned relates to refining of metals according to the floating-zone melting techniques.

The main crystalline body of semiconductor structures, such as rectifiers, transistors, thyristors, photodiodes, and the like, consist for themost part of germanium, silicon or an intermetallic compound of Group III and Group V elements of the Periodic System, such as InSb or InAs. In order to manufacture such bodies it is necessary to have on hand large amounts of these semiconductor materials, and these materials must be of extremely high purity and primarily of a monocrystalline structure.

In order to fulfill these requirements to a large extent, there have already been developed diflerent production and processing methods. Thus, it is already known to purify according to the so-called floating-zone melting technique a semiconductor rod derived by being pulled from a body of molten metal or by depositing of semiconductor material on a monocrystalline semiconductor core. In this way it is possible to achieve materials which have been purified to the desired extent and these materials can be produced in the required amounts in this way.

According to these latter methods the material of the rod is placed in a molten condition at a localized area for example by heating the rod at the localized area by means of an induction coil supplied with high frequency alternating current. This localized zone of molten material is then shifted a number of times along the entire length of the rod. Treatment of the rod in this way results in enriching the molten zone of the rod with impurities while the recrystallizing material of the rod becomes depleted in its content of impurities. By shifting the molten portion of the rod in only one direction axially along the latter, the impurities will be transported in this latter direction to the end of the rod so as to provide the rod with an enrichment of impurities at the latter end thereof.

During treatment of a rod according to this floatingzone melting technique, it is important to maintain and even improve monocrystalline structure of the semiconductor rod.

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Apparatus for carrying out floating-zone melting of rods is of course known in which the semiconductor rod is surrounded by an induction coil and is supported in a suitable holder at one end while at its other end the rod is fused, by means of the induction coil, with an initially free-standing rod-shaped monocrystalline seed crystal. After the zone of contact between the semiconductor rod and the seed crystal has been melted through, the semiconductor rod and/ or the seed crystal are set into rotary movement about their longitudinal axes. Then the melting zone is shifted in a vertical direction along the semiconductor rod.

It has been found that at the transition or boundary region of the melting zone between the solid and liquid parts of the semiconductor rod different growth rates of the recrystallizing material are encountered throughout the entire cross section of the molten zone if the molten zone is not maintained during the floating-zone melting process as free as possible from vibratory movements of any type. Such different rates of growth of the material which recrystallizes from the molten mass can result in improper crystal formation, particularly in the formation of twin crystals. These undesirable crystals can detrimentally act on the barrier (blocking) and forward conductance of the p-n junctions in semiconductor members made of the material derived from the semiconductor rod.

It is accordingly a primary object of our invention to carry out the floating-zone melting method in such a way that during treatment of a rod according to this latter method the rod is insulated from mechanical vibrations which might result in undesirable recrystallization of the molten material.

In particular, it is an object of our invention to provide an apparatus which will insulate the rod from mechanical vibrations of any type during treatment of the rod according to the floating-zone melting technique.

The objects of our invention especially include the provision of guiding and bearing structures which by their combination and arrangement, according to our invention, will result in a vibration-free treatment of the rod enabling improper crystal formation to be reliably avoided.

Our invention is illustrated by way of example in the accompanying drawings which form part of our application and in which:

FIG. 1 is a partly schematic side elevation of one possible embodiment of an apparatus constructed according to our invention for practicing the method of our invention;

FIG. 2 is a partly broken away front elevation of the apparatus of FIG. 1;

FIGS. 2a and 212 show partly sectional side and top views, respectively, of a modified embodiment;

FIG. 3 is a sectional plan view taken along line III-III of FIG. 1; and

FIG. 4 is a sectional elevation of the structure shown in FIG. 3, taken along line IVIV of FIG. 3 in the direction of the arrows.

The apparatus of our invention illustrated in FIGS. 1 and 2 includes a support means made up in part of a rigid standard 2. This standard 2 is hollow and made of a material of high specific weight, such as, for example, steel. The standard 2 may have a height of 4 m., for example. The support means of our apparatus includes in addition to the standard 2, a primary foundation 3 made in the form of a robust concrete block and situated on the floor of the shop. The standard 2 is provided with a pair of parallel vertical guide rails 6 the cross-sectional configuration of which is apparent from FIG. 3. These rails 6 form part of a guide means of our invention and may be made of a material such as hardened steel. The guide means which is formed partly of the rails 6 serves to guide a carriage means composed of a pair of carriages 7 and 8 which are guided for vertical movement. The carriage 7 is shown in a sectional plan view in FIG. 3 and in vertical section in FIG. 4. The carriage 8 has a construction identical with that of the carriage 7.

The standard 2 fixedly carries supporting arms 9 and which project forwardly from the standard with the arm 9 situated beneath the arm 10, as is apparent from FIGS. 1 and 2. These arms fixedly carry a container 41 which can either be evacuated or can be filled with a suitable inert or protective gas. Thus, this container is situated at the front side of the standard 2. The carriages 7 and 8 serve as a support for a rotary shaft means formed by the coaxial vertical shafts 13 and 14, which are also supported for rotary movement by the arms 9 and 10 and which extend through gas-tight seals into the interior of the container 41 which is provided with an inspection window. The ends of the shafts 13 and 14 which are situated within the container 41 carry holders 26 and 27 for the crystalline rod 28 which forms the work material treated with the apparatus and method of our invention.

This rod 28 is made of a semiconductor material such as, for example, silicon, and it is this rod 28 which is subjected to the floating-zone melting process. In addition, there is situated within the container 41 a stationary heating coil 23 which surrounds the crystalline rod 28 and which is energized with a high-frequency alternating current.

A motor means is operatively connected with the rotary shaft means 13, 14 for rotating the latter, and this motor means is formed by a pair of motors 13a and 14a which serve through transmissions described below to drive the Shafts 13 and 14, respectively. These motors are especially balanced so as to be free of vibrations and all rotary parts of the motors are supported for rotation by play-free bearings which may take the form of suitable journals or slide bearings. The motors 13a and 14a drive the shafts 13 and 14, respectively, through worm-drives 15 (FIG. 1) made up of especially constructed worms and worm wheels which are free of vibrations. The motors 13a and 14:: are respectively carried by the carriages 7 and 8 which are in turn vertically displaced by way of a spindle means formed by a pair of ball-bearing spindles 16 and 17. These spindles 16 and 17 are driven through a transmission 19 which is in turn driven from a plurality of shafts linked together and including the shaft 20 linked by the universal joints 21 to the other shafts. The system of linked shafts is driven through a step-down transmission 22a by a driving motor 22 (FIG. 1). Before the floating-zone melting process is initiated, a monocrystalline seed which is fixed in the lower holder 27, for example, is fused with the crystalline rod 28 which initially is fixedly carried only by the upper holder 26. This fusing of the monocrystalline seed to the rod 28 is brought about by means of the heating coil 23. At the instant when the entire cross section is melted through at the zone of contact between the seed and the rod, the shafts 13 and 14 are set into rotary movement. Then the carriages 7 and 8 are moved in unison in a downward direction, so that in this way the molten zone which is maintained at a localized portion of the rod 28 is shifted downwardly along the rod 28.

In order to maintain the liquefied molten zone of the rod 28 free from vibrations, our invention includes, as one of its features, the provision of the worm-drive transmissions 15 between the motors 13a and 14a and the shafts 13 and 14, these transmissions being substantially free of any play and being carried by the carriages 7 and 8 between the motors 13a and 14a and the shafts 13 and 14, as pointed out above. It is of particular advantage to use for the play-free worm-drives 15 commercially available worm gearing known in the trade under the trademark Cavex worm gears. These transmissions have worms provided witha concave flank profile.

The spindle means which is operatively connected to the carriage means 7, 8 for displacing the latter is formed by the pair of spindles 16 and 17, as pointed out above,

and these spindles, in accordance with a further feature of our invention, are in the form of play-free ball-bearing spindles. In other words the threads of the spindles 16 and 17 are constructed in such a way that they can be filled with spherical members which can freely roll along the thread grooves, and in this way ball-bearing spindles of almost no capability of setting up or transmitting the vibrations are provided. At the ends of the spindles the spherical ball-bearing members guided along the threads thereof are returned through suitable passages so as to again move along their initial path of helical movement along the spindle members. The return elements for the spherical members of the ball-bearing spindles are con structed so as to interfere in no way with the nuts 16a and 17a respectively carried by the carriages 7 and 8 and cooperating with the spindles 16 and 17. These ballbearing spindles 16 and 17 are, according to a further feature of our invention, supported for rotary movement in vibration-free bearings 16!) and 17b which preferably take the form of slide bearings. The advantage of using ball-bearing spindles of the above type is not only that they are practically free of any play, but in addition they are very easy to set into and maintain in movement. Also, their direction of movement can be reversed without any backlash, so that it is possible to move the carriages 7 and 8 through the spindle means of our invention without any backlash and in a completely vibration-free manner so as to guarantee insulation of the crystalline rod 28 from any vibrations which might induce undesirable vibratory movements in the melting zone. Commercially available ball bearing spindles known by the trademark Rotax ball-bearing spindles have proved to be particularly suitable for the structure of our invention.

In order to provide for guiding of the carriages 7 and 8 in a manner which is as free as possible of vibrations, the guide means of our invention, which includes the rails 6, as pointed out above, has a construction which is particularly free of play and which at the same time renders the carriages very easy to move. This guide means includes the pre-stressed ball-bearing guides 18 (FIGS. 3 and 4) which are stressed in the sense that they act with a given pressure on the spherical bearing members. These prestressed guides 18 for the ball members have proved to be extremely free of any play while at the same time guaranteeing that the carriages can easily be set into movement. Therefore, this construction also contributes to the insulation of the crystalline rod 28 from vibrations which might otherwise be set up particularly at the molten zone of the rod. As is particularly apparent from FIGS. 3 and 4, each of the pre-stressed ball-bearing guides 18 is made up of a pair of pre-stressed ball passages 18a cooperating with one of the rails 6 and communicating at their upper and lower ends with semi-circular ball passages which in turn communicate with non-pre-stressed return passages 18b in which the balls move freely without any of the pressure applied thereto as in the case of the prestressed guide passages 18a. Thus, the guide means includes the endless passages formed by the ball guides 18a and 18b and the semi-circular portions interconnecting these guides at their ends, and it will be noted that each rail 6 has along its outer edge a fin or rib extending between a pair of rows of the ball members, these rows being maintained separate from each other at each of the guides 18. In each row the balls are arranged along an endless path as is apparent from FIG. 4, and these balls continuously circulate along this endless path to provide the vibration-free guiding of our invention.

In order to further contribute to the insulation of the molten rod material from influences which might set up vibrations in the molten material, the shafts 13 and 14 are supported for rotary movement by bearings 11 and 12 which preferably take the form of slide bearings which are free of any play and which are carried by the car riages 7 and 8 as indicated for the upper carriage 7 in FIG. 1. It is also of benefit, in accordance with our invention, to provide in the arms 9 and 10 suitable vibrationfree slide bearings for the shafts 13 and 14. As was indicated above the motors 13a and 14a which are carried by the carriages 7 and 8 are especially balanced so as to be free of vibrations and in addition the rotary shafts of these motors are supported for rotation in vibration-free slide bearings.

In order to further contribute to the insulation of the rod 28, particularly at its molten Zone, from influences which might undesirably set up vibrations in the rod 28, the transmission of vibrations from the shafts 13 and 14 to the rod 28 is avoided by driving the ball-bearing spindles 16 and 17 from a play-free worm-drive means 19 situated in the lower part of the standard 2. Also for this purpose it is desirable to use a play-free worm drive available on the market and known as Cavex worm drives. The transmission of motion from the driving motor 22 to the worm drive means 19 is brought about, as pointed out above, by a series of shafts which include the shaft 20 and which are linked to each other by universal joints 21. In accordance with a further feature of our invention these universal joints 21 include elastic motion-transmitting members made of suitable plastic discs, so that no jarring impacts or vibratory movements can be transmitted from the driving motor 22 to the floating-zone melting apparatus. This driving motor 22 whose speed of rotation must be controlled and from which therefore it is possible to derive undesirable vibrations is separated from the primary foundation 3, together with the stepdown transmission 22a, by being mounted together with the latter on a secondary foundation 29 situated on the floor of the shop and located at a place separate and spaced from the primary foundation 3. In this way any vibratory movements situated at the motor 22 or the transmission 22a will be prevented from reaching the foundation 3 and thus from reaching the floating-zone melting apparatus. The structure includes an unillustrated control and service desk where the operator will manipulate control elements so as to control the operation of the apparatus, and in accordance with a further feature of our invention this latter desk is mounted on a further unillustrated secondary or auxiliary foundation 42 of the control or service desk or cabinet 43, which is separate and spaced from the foundation 3, so that manual movement of manually engageable elements or any movable control elements will be prevented from inducing vibrations in the molten zone of the rod during the crystalpulling process. The primary foundation 3 can, in the event that it is necessary to shield the apparatus from extraneous sources of vibration, take the form of a concrete block 44 (FIGS. 2a, 2b) which is mounted on a spring assembly which will damp out any vibrations to prevent them from reaching the concrete block of the support means of our invention.

As shown in FIGS. 2a and 2b, the concrete block 44 is located in a pit lined with concrete 47 and is supported at four lugs 49 only. Each lug 49 is fastened to a vertical rod 50 whose upper end is seated on a spring 45 abutting against two rigid bases 48. Preferably a dashpot or shock absorber is mounted on each rod 50 between the spring 45 and the lug 49. A plate 51 covers the pit.

The stationary heating coil 23 which is situated within the container 41 can, for example, be supplied with a high frequency alternating current having a frequency in the range of between 1 and megacycles (m.c.p.s.), and a suitable HF-generator may be used for this purpose. The use of such a generator involves the danger of exciting in the molten zone of the crystalline rod 28 vi brations resulting from modulation of the high frequency heating current with low frequency alternating current. This latter danger can be avoided by using a high frequency heating current whose ripple amplitude is considerably below 1%. In the event that the d1rect current required for the high frequency generator is derived by rectification of alternating current, it is possible to achieve this later maintenance of the ripple amplitude below 1% by smoothing the direct current with extreme care.

As a further feature of our invention contributing to insulation of the rod 28 from vibrations, it is of advantage to make the standard 2 hollow, so that it has a free interior space 4, and this space can be filled with a granular material 4a of high specific Weight, such as, for example, dry sand, so that the occurrence of vibrations in the standard 2 is avoided and in this way any vibrations of the standard 2 are damped.

In order to damp out any low frequency vibrations which still might be received by and occur in the carriages 7 and 8 as well as in the standard 2, the carriages 7 and 8 are preferably provided with additional vibratory auxiliary mass systems 24, and the upper end of the standard 2 is provided with a similar auxiliary system 25.

The various features of our invention described above and illustrated in the drawings, insofar as they are not heretofore known, are of great merit and provide a great advance in the art both individually and in their combination with each other.

We claim:

'1. Apparatus for refining a rod according to the floating-zone melting technique, comprising rotary shaft means adapted to carry and rotate a rod during subjection thereof to the floating-zone melting technique, play-free wormdrive means operatively connected to said rotary shaft means for rotating the latter, carriage means carrying said rotary shaft means, ball-bearing guide means guiding said carriage means for movement, and rotary ballbearing spindle means operatively connected to said carriage means for displacing the latter, so that the rod is maintained free of mechanical vibrations during treatment of the rod according to the floating-zone melting technique and including a hollow standard carrying said spindle means and said guide means, said hollow standard being adapted to be filled with a granular material for increasing the stability of the apparatus so as to further contribute to the vibration-free treatment of a rod.

2. Apparatus as recited in claim 1 and wherein said hollow standard is filled with dry sand.

3. Apparatus for floating-zone melting a rod of crystallizable material comprising a processing container, a pair of holder means for vertically holding a rod of crystallizahle material therebetween, said pair of holder means mounted in said container and extending vertically upwardly and downwardly, respectively, through openings in opposing walls of said container, a stationary heating device mounted in said container coaxially with said pair of holder means so as to surround a rod of crystallizahle material held by said pair of holder means for forming a molten zone therein, means for displacing said pair of holder means in axial direction thereof, said displacing means comprising a pair of vertically displaceable carriages located one above and the other below said processing container, each of said carriages being firmly connected respectively with one of said pair of holder means, ball-bearing spindle drive means operatively connected to said carriages for displacing the same, said ball-bearing spindle drive means including a pair of drive spindles formed with a thread groove filled with ballbearings, driving motor means, connecting members interconnecting said driving motor means and said drive spindles, and slide bearing means for mounting therein said drive spindles and said connecting members.

4. Apparatus according to claim 3 wherein said pair of end holder means are exially rotatable, and including an additional driving motor mounted on each of said carriages and a respective worm drive mounted free of play on each of said carriages and operatively interconnecting said additional driving motor to the respective holder means for rotating the latter.

5. Apparatus according to claim 3 wherein said connecting members interconnecting said motor means and said drive spindles comprises rotary shafts, and play-free worm drive means operatively interconnecting said rotory shafts and said drive spindles.

6. Apparatus according to claim 3 wherein said connecting members interconnecting said motor means and said drive spindles comprises a respective step-down transmission operatively connected to said motor means, a respective play-free worm drive connected to each of said drive spindles, and a universal drive shaft interconnecting said transmission and said drive shaft.

7. Apparatus according to claim 6 wherein said universal drive shaft comprises a rigid shaft portion having at both ends thereof, respectively, a universal joint formed of elastic discs of plastic material, a driving rotary shaft shaft operatively connecting the other of said universal joints with one of said drive spindles.

8. Apparatus according to claim 3 wherein said heat- 8 ing device comprises an induction coil energizable by a high frequency alternating current.

References Cited UNITED STATES PATENTS 3,030,194 4/1962 Emeis 23--273 3,144,308 8/1964 Tarter 23273 3,211,881 10/1965 Jablonski 23273 3,238,022 3/1966 Miller 23273 3,337,303 8/1967 Lorenini 23273 2,975,036 3/1961 Taylor et a1. 23273 3,046,808 7/1962 DeMart 74424.8

NORMAN YUDKOFF, Primary Examiner connected to one of said universal oints, and a driven 15 S. S. SILVERBERG, Assistant Examiner U.S. C1.X.R. 74-424.8 

